This invention relates to apparatus for displacement washing of fibrous material suspended in a liquid and is particularly adapted for washing fibrous, cellulosic materials that have been treated in a pulp digester.
Cellulose fibers are liberated and obtained from wood pulp and other cellulosic materials by cooking a suspension of the material in a liquid, typically in a pulp digester. The suspension liquid is then separated or washed from the fibers to the maximum extent possible.
One known technique for washing a suspension of cellulose fibers is by passing a washing liquid through the fibrous material to displace the suspension liquid. With batch digesters, displacement washing is often accomplished by admitting washing liquid directly into the digester upon completion of cooking, thereby washing the full contents of the digester. A later development involves the use of so-called diffusers, in which a thick bed of fibers, after removal from the digester, is purged of suspension liquid over a prolonged period of time by a washing-liquid flow.
Washing by displacement is also utilized in connection with continuous pulp digesters, with the digester normally being elongated at its lower end to provide a washing zone downstream of the cooking zone. The washing liquid is usually introduced at the bottom of the washing zone and flows generally upward through the digested chip material as it proceeds towards the digester outlet.
A principal disadvantage of displacement washing, as heretofore practiced, is that it is difficult to achieve uniform distribution of the washing-liquid flow throughout the full length and depth of the defibered chip material. As cellulose material, for example, wood chips often varies considerably as to digestability owing to various factors such as habitat, size, shape, etc., small zones of slightly different degrees of digestion, and hence of differing resistance to the flow of liquid therethrough, frequently exist within the digester. The result is that canals tend to be formed within the material bed by the washing-liquid flow. Consequently, the material is not uniformly purged of suspension liquid. The likelihood of canalization occurring increases with the thickness of the bed within the digester since the washing liquid must travel along correspondingly lengthened flow paths.
Moreover, the risk of canalization remains even if the digested chips are transferred to a separate vessel for washing, and this notwithstanding that the chips are defibered to a great extent upon removal from the digester. Here, uneven packing of the material within the vessel may lead to the formation of canals, or a certain degree of flow canalization may also occur along the smooth side walls of the vessel. Non-uniform distribution of the washing liquid results; and those areas of the bed of higher resistance to liquid flow are not satisfactorily washed.